The present invention relates to a system for controlling curls of a paper. More particularly, in an image forming apparatus in which a visible image provided on a photoconductive element is transferred to a paper being transported along a predetermined transport path, the present invention is concerned with a system for sensing curls of the paper and correcting the curls on the basis of their particular conditions.
In an electrophotographic copier, facsimile apparatus, electrostatic printer or like image forming apparatus which uses an electrophotographic process, an electrostatic latent image or a toner image provided on a photoconductive element is transferred by a transfer charger to a paper which is transported along a predetermined path. The paper carrying the image thereon is separated from the photoconductive element and then fed to a fixing station which is adapted to fix the image by applying heat to the paper. While the paper is driven through the fixing station, it is deformed or curled due to the heat and the decrease in its water content which is ascribable to evaporation. Generally, a paper for such an application is provided with a water content of about 6% at the production stage and wrapped in an elaborated manner to be sealed from moisture. In addition, a paper feed tray of a copier is provided with a hermetic configuration and even furnished with a heater for controlling relative humidity.
However, when the papers are left in an unwrapped condition as often occurs due to incomplete management and if the humidity is high, they absorb moisture and, when used in the moist condition, suffer from curls. Should such a curled paper be subjected to any subsequent step, there would occur various problems as enumerated below.
(1) When a paper is curled, its contact with a photoconductive element at a transfer stage become unstable resulting in irregular image transfer, local omission of an image, and other occurrences which are undesirable for stable reproduction.
(2) A paper with relatively great curls cannot be surely separated from a photoconductive element and, in the worst case, causes an image to be partly re-transferred to the photoconductive element, critically degrading the image quality. These occurrences are observed during the course of separation process which belongs to a sequence of image forming processes. Specifically, incomplete separation of a paper from a photoconductive element is apt to occur when an electric current which flows from a separation charger into a paper is small. Conversely, re-transfer of an image is apt to occur when the separation current is large. Although a separation current is usually so adjusted as to eliminate such incomplete separation and re-transfer, the image quality attainable with a curled paper is poor because the value of separation current which causes incomplete separation and re-transfer is partly different from the usual.
(3) A paper separated from a photoconductive element carries a non-transferred toner image thereon and therefore should not be nipped by a transport roller pair from opposite sides. It is a common practice to transport such a paper to a fixing station by using a suction assembly which retains the paper on a belt by suction. However, when a paper separated from a photoconductive element and driven toward the fixing station is curled, its curled portions fail to be sucked depending upon the conditions of the curls. Those curled portions so transported afloat are liable to abut against other units to become dog-eared or to jam a transport path.
(4) In a two-side copy mode, papers each being provided with an image on one surface thereof are individually turned upside down by an inverting mechanism and sequentially stacked on a two-side tray. Then, after a document has been replaced with another, those papers are fed again to reproduce an image on their other surface, or back. When papers carrying images on their one surface are curled at a fixing station, they cannot be stacked smoothly on the two-side tray or re-fed smoothly therefrom. This constitutes a cause of jams, scratches on copies, dog-earing, positional deviation between an image and a paper, etc.
(5) In a combination copy mode, papers each undergone the first copying operation are sequentially stacked on an intermediate tray with or without the intermediary of an inverting mechanism. After the replacement of a document, those papers are fed again for the second copying operation. When any of the papers is curled at the fixing station during the first copying operation, it is transported to the fixing station in a curled condition. Assuming that the fixing station is implemented with a heating roller and a pressing roller which is pressed against the heating roller, the curled paper which is driven toward a nipping section of such a roller pair is partly raised at its leading edge away from an inlet guide and therefore fails to smoothly enter the nipping section. Such a paper is apt to be creased or to jam the transport path. In the light of this, there has been proposed to provide a narrow slit between one of the heating and pressing rollers and the guide member (Japanese Laid-Open Patent Publication (Kokai) No. 53-35731), or to provide two ridges at opposite sides of an intermediate portion of a guide surface of the inlet guide so that a paper may be nipped by the roller pair while being stretched toward opposite sides (Japanese Laid-Open Patent Publication (Kokai) No. 60-50568). Although the slit scheme mentioned above may effectively prevent a paper from being creased, it is incapable of eliminating a jam when the curl or the thickness of a paper is too great to allow the paper to smoothly move through the slit. The ridge scheme is not effective unless a paper is curled in a certain limited direction and rather aggravates creasing when a paper is curled in the opposite direction.
(6) When a sorter, finisher and other post-processing units are associated with a copier, a curled paper coming out of the copier is apt to jam such units or to impair their functions.
To solve the problems (1) to (6) discussed above, a variety of curl correcting approaches have heretofore been proposed such as a one which applies a stress or heat to a paper which is curled in a predetermined direction in the opposite direction to the curl (Japanese Laid-Open Utility Model Publication (Kokai) No. 59-30156), and a one which senses a curled condition of a paper, selects one of two different transport paths based on the curled condition, and applies a stress or heat to the paper in the opposite direction to the curl (Japanese Laid-Open Utility Model Publication No. 61-28754). All these prior art curl correcting schemes is capable of straightening a paper with no regard to the direction of a curl and adjusting the amount of correction on the basis of the amount of curl.
However, the amount and direction of curl depends upon the kind of a paper, water content of a paper, ambient conditions of a transport path, etc. Hence, even if a curl correcting device is so adjusted as to fully straighten a paper at a certain instant, a change in the kind of a paper to be transported and/or in the ambient conditions causes the amount of curl of a paper before correction to be changed. In this condition, a curl may remain even after the correction. Moreover, the direction of curl may become even opposite to the expected direction and, in this case, the curl correcting device simply serves to increase the amount of curl.
As discussed above, the prior art curl correcting devices are incapable of coping with all the kinds of curls without any assistance. In addition, they cannot deal with all the kinds of curls without resorting to a substantial degree of stress or heat and therefore an expensive and complicated construction.